The science behind overheating

We understand that we heat up when we exercise but have you ever stopped and thought why this happens or considered how it could affect your performance?

We humans are not very efficient when it comes to how much energy we create and how much of this energy can be used for performance. Cycling, which is said to be one of the most efficient forms of exercise, can achieve up to 30% efficiency. This means that producing 300 watts on your power meter you need to create 1000 watts of energy. But it could be worse. Running is one of the most inefficient forms of physical activity, with an estimated efficiency of only 5%.

Creating power and heat in the body

So, if it takes 1000 watts input to make 300 watts output - where do the 700 watts go to? First, you should understand that watts are measured as power/energy over time and many things can be measured in watts over time, including a microwave oven, light bulbs, and automobile engines. All these appliances also create heat, which means that the missing 700 watts aren't really gone. They are primarily converted to heat during the energy transfer stage, and that is why you get hot during exercise. Getting hot is a result of our body’s incredible but inefficient thermoregulation system. 

But there is more to it than that. Creating power comes from energy converted from stored sugars, fats, ATP, synchronized with muscle contractions which are in turn fuelled with oxygen via blood flow - a very complex system. So now we know that the side effect of this power creation is heat, but how do we cool down after heating up?

Body cooling and hydration

Heat is removed from the body by cooling down the blood which is near the surface of the skin. The first step is that your body will start to divert more blood to the skin’s surface using a vasodilation mechanism - where the blood vessels open to allow more blood flow to these areas.

Simultaneously, the sweat glands in the skin start to produce sweat which quickly evaporates when it comes in contact with air flow along the skin. The evaporation process via evaporation of water particles on our skin surface cools the body by removing heat produced by the muscles and represents the primary way we lose heat.
The blood is cooled by circulation through the body and continues to remove heat energy, in a way similar to how water is circulated in a car engine and radiator for cooling. Your radiator is the skin, the largest organ of your body.

This cooling process requires water for sweat which is often referred to as hydration. Staying hydrated could be better described as keeping enough water reserves for cooling.  Being under-hydrated or dehydrated means insufficient water supply is available for cooling. Dehydration means less efficient cooling, which leads to getting hotter faster. In other words, hydration, cooling, and core body temperature are directly related to one another, and insufficient cooling means performance goes down, as shown in the graph below (2).

The main causes for overheating

Based on the facts explained above, we can easily understand the main reasons for overheating:

1. Creating more power (heat energy) than your body can remove.
2. Dehydration impairs the cooling part of your body’s thermoregulation processes.

Removing heat from the body is a slow process. Blood needs to circulate close to the skin where it cools down before going back to cool the internal organs. The lag between heat generation and cooling process implies that your body needs to have a heat reserve: a certain amount of excess heat that your body can absorb. If we exceed that amount, the body starts going into a protective, avoid-overheating, mode. But what is the link between the heat reserve and core body temperature? It’s quite simple: when the former is activated, the latter will increase, but at a slower rate. In the graph below (1) you can see how the heat loss is lagged compared to heat production. That lag is the heat reserve in your body and in this case, your body is producing more heat than it can remove. When both lines meet, the body’s thermoregulation is in balance.

Another interesting physiological fact is when the cooling system is being utilized, blood flow is being directed towards the skin for cooling – but more importantly, blood is being diverted away from the muscles used for power generation. So, in heightened cooling mode, less oxygen is available for the power generation muscles. Put simply, when heating up, the power output needs to be reduced to compensate for the oxygen reduction in the muscles. The hotter you get the more your body compensates, and your performance goes down.

The consequences of overheating

It makes sense that overheating or heat stress is not good. Before getting to this point, the athlete has already lost a great deal of performance with the cardiac output being redirected (up to 70%) for cooling instead of power production. Once the heat stress state has started, it now starts to affect your cognitive ability, sense of balance, and motor skills. This is when it starts getting dangerous. 

Your body starts to activate defensive mechanisms to shut down the heat sources to prevent serious injury, or in some cases death. After overheating, several events happen to the body, which can take a long time to recover and continue to perform at a high level. The body somehow remembers the heat stress state and tries to prevent getting into a similar situation again. There are many stories of athletes taking months if not even over a year to 'fully recover' and perform at competitive levels again.

Next time you start to see the sweat forming on your forearm - have an appreciation for your incredibly complex cooling system and understand that the sweat you see as well as the cool feeling on your skin as it evaporates is directly related to your performance levels.

Scientific references:

(1) Andreas D. Flouris (2019): Human Thermoregulation, in Heat Stress in Sport and Exercise (Julien D. Périard • Sébastien Racinais Editors), Springer, p. 3-28.

(2) Louise M. Burke (2019): Hydration in Sport and Exercise, in Heat Stress in Sport and Exercise (Julien D. Périard • Sébastien Racinais Editors), Springer, p. 113-138.